Injection molding is a manufacturing process used to produce plastic products of all sizes. Typically, the process involves heating thermoplastic or thermosetting plastic materials until they are in molten form and injecting the molten plastic into a mold cavity. Inside the mold cavity, the plastic material cools and hardens, taking on the form of the mold cavity.
Injection-molding systems typically include a “material hopper” which holds the raw plastic and feeds the raw plastic into a heated “injection barrel” which turns the plastic into a molten state. The injection barrel includes a plunger, screw, or other means to pressurize and force the molten plastic into the mold through a nozzle located at the end of the injection barrel. The nozzle typically connects to the mold by means of a “sprue”, which is a bushing that forms a seal between the nozzle and the mold. Once molten plastic enters the mold, it flows into the mold cavity by means of channels in the mold, which are generally referred to as “runners.” A “gate” defines the entrance of each runner into the mold cavity. Various types and sizes of gates may be used depending upon the design of the system.
In some injection-molding systems, non-plastic inserts may be encapsulated within, and molded into, the final plastic product. For example, electrical conductors, such as wires or strips of electrically conductive materials, may be encapsulated within portions of specialized plastic components (e.g., automotive and aircraft plugs and connectors).
Due to the high pressure of molten plastic flowing through the cavity (25-250 tons per square inch), inserts must generally be fixed in their desired positions within the mold cavity; otherwise, the pressurized molten plastic flowing through the cavity will move the inserts in an undesirable manner. In the case of electrical inserts, fixation of the inserts is particularly important. The electrical inserts cannot be allowed to contact each other due to the risk of creating a short circuit. Additionally, in many end-use applications, great care must be taken to ensure that the electrical inserts are fully encapsulated within the plastic part in order to prevent contamination and/or oxidation from air, moisture, or oils.
A multi-shot injection molding process is a common method employed for the manufacture of plastic components with electrical inserts. In a typical two-shot process, the pressurized plastic injection into the mold is performed twice, with an interruption in between the shots. Fixation of inserts during the first of the two injections is traditionally accomplished by means of removable, small diameter hold-down pins. Typically, the small pins are inserted through holes in the mold and into the mold cavity. The pins may be configured to put pressure on the surface of an electrical conductor in order to prevent vertical movement, or the conductor may be enlarged in certain sections and configured with a through-hole in those enlarged sections, such that the pins may be inserted through the conductor to prevent lateral movement of the conductor.
After the first injection is complete, and the mold cavity is filled, the plastic within the mold cavity is allowed to cool and harden. Once the plastic in the mold cavity sufficiently hardens, the pins are removed from the mold cavity, leaving an unfilled opening (i.e., a pinhole) within the mold cavity where each pin was removed. Accordingly, a second injection of molten plastic is required to fill the openings left when the pins are removed.